Hi @Landjah, welcome to our forum!
If you start the device up without a SIM card and then open up the Phone application > type in *#*#7378423#*#* > Service info > SIM lock.
How does it look in here? Does it show [X] Network 0?
The X means that the device is locked to a specific carrier and the 0 means that there are no unlocking attempts left. If it does not say 0, you have x amount of tries left (if it shows 1, you have 1 try left). Double check with your operator to make sure that the 16 digit NUC code you have received from them is the correct one, so you don't waste any more tries.
Let me know how it goes! 
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